Extending aircraft- and tower-based CO2 flux measurements to a boreal region using a Landsat thematic mapper land cover map

There has been an increasing need to measure the exchange of CO2 between th e atmosphere and vegetated surfaces for large areas in order to quantify th e carbon budget of the terrestrial biosphere. The boreal landscape is heter ogeneous owing to different forest cover types and disturbance regimes, and regional quantification of CO2 flux is difficult without numerous species- specific flux measurements. During the Boreal Ecosystem-Atmosphere Study in 1994 and 1996 the National Research Council of Canada operated a Twin Otte r aircraft that measured CO2 sensible and latent heat fluxes, and other tra ce gases over boreal forests in Saskatchewan, Canada. A flux-unmixing metho d was developed to calculate flux densities for eight major cover types fro m the aircraft-based measurements. Using a coregistered land cover map at 3 0-m resolution derived from Landsat thematic mapper data, the contribution of each cover type to the CO2 flux measured by the aircraft was estimated u sing a contributing area (footprint) function according to the wind directi on, the atmospheric stability, the horizontal distance of each pixel from t he aircraft, and aircraft height. The unmixing method uses a linear inversi on method with the footprint-weighted cover type fractions as the set coeff icients for each segment of a flight line. In the inversion, various constr aint strategies were used to confine the inversion results to minimize the effect of various sampling errors. It is shown that (1) mathematical constr aint is critically important in the inversion, (2) a simple constraint towa rd the mean flux values is effective in producing reasonable inversion resu lts, and (3) the inversion accuracy can be further improved when simultaneo us tower measurements in the dominant cover types are used as tight constra ints. With such constraints the estimated fluxes from the cover types witho ut tower measurements appear to be reasonable. It is concluded that aircraf t measurement adds to our ability to map the regional flux field using remo te sensing images because (1) it allows the derivation of flux data for cov er types without tower-based measurements and (2) it can be used to infer t he representativeness of tower measurements for the measured cover types in the landscape.